Xerographic charging apparatus



Dec. 11,1962 J. J. CEODICHINI 3,068,356

XEROGRAPHIC CHARGING APPARATUS Filed June 15, 1960 2 Sheets-Sheet 1INVENTOR. JOSEPH J. CODICHINI A 7' TORNEY Dec. 11, 1962 J. J. CODICHINI3,058,356

XEROGRAPHIC CHARGING APPARATUS Filed June 15, 1960 2 Sheets-Sheet 2:

8 ORONOD E/ SCREEN 5 CORONODE VOLTS x I03 SCREEN VOLTS x :0

Va l6 /4 %6 SPACING IN INCHES BAREPLA'TE CURRENT AMPS x 10' CORONODEVQLTAGE KV F/ 4 INVENTOR.

G JOSEPH J. CODICHINI BY fa/M A7 TORNE Y United States Patent Office3,058,356 Patented Dec. 11, 1962 3,968,356 XEROGRAPHEC (IHARGHN GAPPARATUS Joseph J. Codichiui, Fairport, N.Y., assignor to XeroxCorporation, a corporation of New York Filed .lune 15, 1960, Ser. No.36,352 1 Claim. (Cl. 25049.5)

This invention relate to improved electric circuit means for controllingthe application of electrostatic charge from a corona generating deviceto a xerographic plate. More specifically this invention relates toimproved and simplified electric circuit means for controlling theoutput of a corona generating device whereby required surface potentialon a xerographic plate is effected with a minimum of compensatingadjustment.

In the process of xerography, for example, as disclosed in CarlsonPatent 2,297,691 issued October 6, 1942, a xerographic plate comprisinga layer of photoconductive insulating material on a conductive backingis given a uniform electric charge over its surface and is then exposedto the subject'matter to be reproduced, usually by conventionalprojection techniques. This exposure discharges the plate areas inaccordance with the radiation intensity that reaches them, and therebycreates an electrostatic latent image on or in the photoconductivelayer. Development of the latent image is effected with anelectrostatically charged, finely divided material such as anelectroscopic powder that is brought into surface contact with thephotoconductive layer and is held thereon electrostatically in a patterncorresponding to the electrostatic latent image. Thereafter, thedeveloped xerographic powder image is usually transferred to a supportsurface to which it may be fixed by any suitable means.

In the art of xerography it has been found that consistent reproductivequality is best effected when the uniform potential applied to thephotoconductive surface of a xerographic drum is maintained at aconstant value, considered optimum when in the range of approximately600 volts. In xerographic machines currently in wide commercial use,this potential is applied by a corona generating device, commonlyreferred to as a scorotron, which is arranged in closely spaced relationto a xerographic plate which may be in the form of a cylindrical drum.It is known in the art that for any value of vol age supplied to thescorotron the resulting corona emission applied to the drum surface toeifect potential thereon is partially a function of the spaced distancebetween the surface to be charged and the scorotron, and for a givenspacing higher supply potential effects higher surface potential.Because of manufacturing tolerances permitted for this spaced distance,and because of variables such as ambient atmospheric conditions andstill others inherent in the scorotron itself which also influencecorona charge imparted, compensating means are required for adjustingthe scorotron supply potential to effect desired surface potential.

The scorotron currently is in wide commercial use and may be of a typedisclosed in Walkup Patent 2,777,957 which generally consists of aplurality of regulating screen wires strung parallel to a conductiveshield, and a plurality of coronode corona discharge wires arrangedparallel to and between the screen wires and the conductive shield.Potential is supplied from a source to the coronode wires which thenemits corona current. When arranged for charging a xerographic plate, aportion of corona emitted is imparted through the regulating screenwires to be applied to the plates photoconductive surface, and excesscorona current is suppressed and drained off by the regulating screenwires. The magnitude of corona imparted from the scorotron is a functionof both coronode potential and screen potential, the screen potentialbeing derived from the corona emission, there existing a nonlinearresistance between the coronode wire and the screen.

Heretofore, it was usual practice to maintain potential on theregulating screen wires equal to potential on the plate surface. Tomaintain equal potentials, a form of electric voltage divider circuithaving compensating variable means was employed whereby the screenpotential could be maintained against variable potential supplied to thecoronode by a balancing of respective circuits. Since coronode potentialand screen potential are each interdependent for effecting surfacepotential, adjustment of one adversely affected the other, and ultimateadjustment of their respective circuits to effect screen potential equalto required surface potential was a delicate and cumbersome task. Inaddition, frequent. failure of the divider circuit was experiencedbecause of excessive demands imposed on the circuit under extremeconditions of operation. Attempts to overcome the cause of circuitfailure by increasing capacity of the circuit components encounteredproblems of physical limitation because of dimensional size of thecomponents. Furthermore, the heretofore used power supplies included thedivider resistor embedded in potting compounds and when the resistorfailed, the power supply had to be scrapped.

Because of the above difficulties, extensive research was conducted fromwhich it was found that reproductive quality of the plate is best whenthe regulating screen wire potential does not exceed surface potentialand quality deteriorates when screen wire potential exceeds surfacepotential.

The principal object of this invention is to improve electric circuitmeans of a corona generating apparatus whereby screen potential does notexceed the potential applied on a xerographic plate.

It is a further object of this invention to improve electric circuitmeans of a corona generating apparatus whereby a single adjustment ofcoronode potential effects a desired potential on a xerographic platewhich equals or exceeds screen potential.

These and other objects of the invention are attained by the means ofthe apparatus of the invention which includes means to effect a variablypreset potential supply to the coronode wires of a corona generatingdevice and a selected resistance in the screen circuit by whichmagnitude of potential generated on the screen does not exceed platepotential.

A preferred form of the invention is shown in the accompanying drawingsin which:

FIG. 1 is a schematic sectional view of a xerographic machine employinga scorotron for applying uniform electrostatic charge to a xerographicdrum.

FIG. 2 is a wiring diagram of the scorotron control circuit of theinvention.

FIG. 3 is a curve typically illustrating variation of coronode potentialand screen potential to obtain plate potential of 600 volts at variablespacing in accordance with the invention; and

FIG. 4 is a curve typically illustrating in accordance with theinvention the relationship of co'ronode voltage vs. plate current at agiven spacing of 1 1 inch using a resistor in the screen circuit ofapproximately 1.5 megohm resistance.

Referring to the drawings there is illustrated in FIG. 1 an automaticxerographic apparatus which may be an adaptation of a type disclosed incopending application Serial No. 837,173, filed August 31, 1959, in thename of Cerasani and Lewis.

Xerographic drum 1 includes a cylindrical member mounted in suitablebearings in the frame of the machine and is driven at a constant rate ina clockwise rotation by motor 18. The drum surface 15 comprises a layerof photoconductivematcrial on a conductive backing 16 (FIG. 2) that issensitized prior to exposure by means of a corona generating device 2which may be of the type mentioned above.

The exposure of the drum to the light image discharges thephotoconductive layer in the areas struck by light whereby there remainson the drum an electrostatic latent image in image configurationcorresponding to the light image of copy to be reproduced. For exposingopaque original copy, apparatus 3 may be employed which may be of a typedisclosed in the above cited copending application in the name ofCerasani and Lewis Whereas for exposing microfilm images, apparatus 4 isemployed which may be an adaptation of a type disclosed in copendingapplication Serial No. 796,561 filed March 2, 1959, in the name of S. R.Johanson.

For exposing opaque copy, copy to be reproduced is set on tray 22wherefrom it is transported on belts 23 being driven by motor 24. Thecopy is illuminated by light 25 and the image is reflected from mirror26 through lens 27 to mirror 28 thence through exposure control 29normal onto surface 15.

In the alternative, for reproducing microfilm images, motor 34 effectstransport of microfilm 35 from supply spool 36 to takeup spool 37. .Atan intermediate transport position, film 35 is illuminated by light 3%;and its image is projected through lens 39 and exposure control 29normal onto surface 15; mirror 28 being pivoted vertically by controlarm 30 so as not to obstruct the optical path when projecting microfilm.

Thereafter the latent image is developed by means of developing unitwhich may be of the type disclosed in copending application Serial No.393,058, filed November 19, 1953, in the names of Mayo et al. in which atwocornponent developing material 45 which may be of the type disclosedin Walkup Patent 2,63 8,416 is cascaded over the drum surface.Developing material, which is stored on the bottom of housing 46, iscarried up by conveyor 47 driven by motor 51 and released into hopper4-8. The developing material then gravitates down, adhering to theelectrostatic latent image on the drum surface. Developer toner '44consumed in the developing step is replenished from storage container 49at a rate determined by the position of control gate 50.

After developing, the powder image is electrostatically transferred to asupport surface web 17 by means of a second corona generating device 7similar to corona generating device 2 mentioned above.

The support surface web to which the powder image is transferred may beof any convenient type and is transported by a handling mechanism 6which may be an adaptation of the type disclosed in Crumrine et al.Patent 2,781,705. The support surface is obtained from supply roll 56and is fed over suitable guide and tensioning rolls and directed intosurface contact with the drum in the immediate vicinity of transfercorona generating device 7. After transfer, the support surface isseparated from the drum surface and guided through a suitable fusingapparatus 8 which may be of the type disclosed in Crumrine et 'al.Patent 2,852,651 whereby the powder image is permanently aifixed to thesupport surface. Thereafter, the support surface is fed over a furthersystem of guide and tensioning rolls and onto a take-up roll 54 that isdriven by motor 55.

After transfer, the xerographic drum surface is cleaned by cleaningbrush assembly having brushes 57 rotated by motor 58 whereby residualdeveloping material remaining on the drum is removed. Suitable lighttraps are provided in the system to prevent any light rays from reachingthe drum surface, other than the projected image, during the period ofdrum travel immediately prior to sensitization by corona generatingdevice 2- until after the drum sur face is completely developed.

Referring particularly to FIG. 2, corona generating device 2 includes atleast one and preferably three coronode wires 67 electrically seriesconnected and arranged parallel to the drum axis of rotation. Potentialis supplied thereto from an available V. AC. source and connected to theapparatus through switch 68. Through continuous variable volttransformer 69, potential is conducted from the secondary side thereofto stepup transformer 70 wherefrom the secondary voltage is connected toA.C.D.C. rectifier 71, which may typically provide half-waverectification of alternating current. Rectified current of coronagenerating potential is then supplied by lead wire 72 to coronode wire67 in the potential range of approximately 6000-9000 volts, the exactvoltage supplied being a function of voltage output adjustably variedthrough variable transformer 60. Capacitor '79 maintains supplypotential substantially ripple-free -at high voltage output of rectifier71. To eliminate shock hazard that would otherwise exist in capacitor 79after opening switch 68 to disconnect scorotron 2 from its supplypotential, bleed wire 81 is provided to shunt capacitor 79 and includesa resistor 80.

With coronode wires 67 energized, corona emanating therefrom ispartially dissipated to conductive shield 73 which is suitably groundedby lead wire 74. Regulating screen wires 75 suppress and drain offexcess corona current thereby to limit and uniformly distribute coronacurrent passing therethrough to surface '15, to apply a uniformpotential thereon. On exposure to light the surface potential iselectrically conducted through conducting plate 16, and lead wire 78 toground.

As stated above, it heretofore was considered necessary that screen wirepotential be maintained equal to surface potential. It is now known thatreproductive quality of the plate is best when the screen wire potentialdoes not exceed surface potential, with notable decrease in printquality at screen potentials exceeding surface potential. To ensure,therefore, that screen potential is limited in value at maximumanticipated potential supplied to the coronode wires, resistor 77 in thescreen circuit is selected having an appropriate resistance. In atypical application of the invention it is sought to maintainapproximately 600 volts on the drum surface requiring an application inthe range of approximately 6000 to 9000 volts to the coronode wireswhich effect a correlated range of variation in current to theregulating screen wires as may be seen in FIG. 3. When employing drumsof length most suitable for 11-inch web width, a resistor 77 having aresistance value of approximately 1.0 megohm has been found to givesatisfactory results whereas for drum lengths most suitable for 24-inchweb width an approximate 0.62 megohm resistor has been found to givesatisfactory results. When employing these resistance values, thedesired results have been achieved for equip ment manufactured withinlimits of manufacturing tolerances with screen potential dropping to aslow as volts and volts respectively for drums of 11-inch and 24- inchweb width at 6000 volt supply potential. In FIG. 4, bare plate current,which is correlated to plate potential is shown plotted against coronodevoltage for a screen to plate spacing of inch.

In operation, with drum 1 in constant rotation, switch 68 is closedenergizing coronode wire 67, and surface 15 is scanned by electrometersensing device 83, which may be of a type commercially available. Thepotential sensed thereby is conducted by lead wire 84 to amplifier '85thence to meter 86, where the magnitude of surface potential isindicated. By adjusting transformer 69, potential supplied to coronodewires 67 is varied until the desired potential on surface 15 isindicated on meter 86.

By the apparatus thus described, there is disclosed electric circuitmeans whereby a uniform potential of desired magnitude is effected onthe photoconductive surface of a xerographic plate by a singleadjustment means. By the circuitry of the invention, the resistance ofthe entire circuit is considerably reduced resulting in an overallreduction of power consumption, which may be as little as one-half ofthat previously consumed with the circuits employed heretofore. Inaddition, the resistors employed are physically smaller requiring lessspace than those used heretofore for voltage dividing and areconsiderably less expensive, as for example, small 10 percent carbonresistors used in ordinary radio circuits can be used. This effects areduced cost of the power supply, especially since the resistor of thescreen circuit can now be mounted external to the power supply.Furthermore, the circuit of the invention eliminates the need forseparate adjustments for charging and screen potentials and simplifiestesting of a power supply.

Whereas the description of the apparatus disclosed herein has been madewith particular reference to a corona generating device 2 employed toeffect desired potential on a xerographic drum, it is apparent that asimilar control may be applied to the corona generating device 7 forapplying a uniform potential to paper web 17 to etfect transfer of thexerographic powder image thereto.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

In a corona generating device for applying a uniform potential on axerographic plate wherein the generating device is arranged in spacedcharging relation to a plate surface and includes a plurality ofregulating screen wires, a grounded conductive shield extending parallelto the regulating screen wires and at least one corona discharge wiresupplied from a potential source and arranged parallel to and betweenthe regulating screen Wires and the conductive shield; the improvementwhich comprises the combination of electric circuit means to apply arequired potential on a plate surface, said circuit means includingmeans to vary the magnitude of potential supplied to said corona wiresincluding an adjustable first voltage transformer, a step-up transformerconnected to the secondary of said first voltage transformer, anA.C.-D.C. rectifier connected to the secondary of said step-uptransformer, a bleeder resistor and a filter capacitor shunting saidrectifier from between which rectified potential is conducted to saidcorona wires, and a resistor connected to the regulating screen Wireswhereby, within the adjustable potential range supplied to said coronaWires, the potential generated on said screen wires is of magnitude notgreater than the magnitude of potential applied to the surface of aplate arranged to be charged.

References Cited in the file of this patent UNITED STATES PATENTS

